High voltage, high current, and high accuracy amplifier

ABSTRACT

A method and system to use voltage isolated and floating differential output amplifiers wired in series and parallel to achieve arbitrary output drive voltage and current for the applications load. Embodiments use multiple voltage-isolated and linearized devices to enable dynamically modifiable, Class A, Class B, and Class AB topologies of predetermined voltage and current performance. Embodiments can correct an output by linearizing one or more devices in a circuit by utilizing a linearization module (e.g., including one or more digital lookup tables, an error simulation circuit, or an equivalent) to linearize at least one parameter of at least one device in the circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of a co-pending U.S. utility patentapplication Ser. No. 11/325,884, filed Jan. 4, 2006, entitled “HighVoltage, High Current, and High Accuracy Amplifier,” to be issued asU.S. Pat. No. 7,365,605 on Apr. 29, 2008. This application also claimspriority from a U.S. provisional patent application Ser. No. 60/641,724,filed Jan. 5, 2005, by the same inventor, entitled “High Voltage, HighCurrent, High Accuracy Solid State Audio Amplifier With AdaptivePerformance Enhancement,” which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to amplifier circuits, and in particular, toproviding a lower distortion amplifier capable of creating high voltagesto drive highly capacitive loads.

2. Description of the Prior Art

With the development of high performance audio formats like SACD andDVD-Audio, new demands are being placed on audio reproduction equipment.Electrostatic speakers provide the highest performance achievable todate. Most electrostatic speakers use conventional low voltage poweramplifiers and step-up transformers to produce the thousands of voltsnecessary to drive electrostatic transducers. These output transformerslimit the signal fidelity of what is achievable with electrostaticspeakers.

Such a design needs to handle the difficult capacitive loading, yetprovide extremely low distortion. Traditional high negative feedbackamplifier designs have a very difficult time producing highly damped,low distortion signals at high currents while maintaining feedbackcontrol, in many applications. Large amounts of negative feedback havetraditionally been used to control the varying current needs for thecapacitance loading. Capacitive loading also increases the powerhandling demands on the amplifier at high frequencies.

The high fidelity requirements of such applications mandate that thelevel of distortion at all frequencies be imperceptibly small. At higherfrequencies the dynamics of the music must be maintained while drivinghigh current capacitive loads. At lower frequencies the currentrequirements are thousands of times smaller because of the higherimpedances. With these extremes occurring simultaneously, the reproduceddynamics are much higher than just what is provided by the input signal.As a result, the amplifier must have much lower distortion thantraditional high performance amplifiers, while handling extremely highimpulse and high frequency currents cleanly. Therefore, the dynamicrange and feedback systems of amplifiers for such applications need tobe many times better than those designed for a traditional amplifierdriving an almost pure resistive load.

One common characteristic of many high negative feedback amplifiers isthat the distortion effectively increases, as the signal level getslower. This is counter to the needs of an amplifier with dynamic rangerequirements of high capacitive loading. This phenomenon is the resultof several processes. One process is the continual “hunting” that isinherent in these designs. The constant correction of the feedbackcircuit induces extra output noise into the output signal. This“hunting” signature of the output noise hides many of the subtle detailsof the input signal (e.g., the music signal, or an equivalent amplifierinput signal). In audio amplifier applications, this output noise canmask the low and mid frequency music detail quite easily.

The large time delays of the large output devices required for the highcurrents in traditional amplifiers, and the delays present in themultiple input stages of the amplifier, can also produce noticeableovershoot and input stage overload, especially at high currents and highfrequencies. This performance is needed in low distortion amplificationof audio recording signals, and also for other applications requiringlow signal distortion by amplifiers.

In view of the foregoing, what is needed is an improved method andcircuit to provide the lowest distortion achievable by any amplifier andcreate high voltages to drive electrostatic speakers or other loads,without the limitations of using large output transformers drivinghighly capacitive transducer loads.

SUMMARY OF THE INVENTION

The present invention can be implemented in numerous ways, such as by amethod, a circuit, or an amplifier system. Four aspects of the inventionare described below.

A first aspect of the invention is directed to a method to produce abipolar output signal from a circuit having at least two stages. Themethod includes coupling an input signal to a first stage of a circuitto produce a first stage output, wherein the first stage of the circuitincludes one or more modules coupled together to produce the first stageoutput; and applying the first stage output as an input signal to asecond stage of the circuit to produce a bipolar second stage output,wherein the second stage of the circuit includes one or more floatingmodules coupled together to produce the bipolar second stage output.

A second aspect of the invention is directed to a method to produce adifferential bipolar output signal from a circuit having at least twostages. The method includes coupling an input signal to a first stage ofa circuit to produce a first stage output, wherein the first stage ofthe circuit includes one or more modules coupled together to produce thefirst stage output; and applying the first stage output as an inputsignal to a second stage of the circuit to produce a differentialbipolar second stage output, wherein the second stage of the circuitincludes one or more floating modules coupled together to produce thedifferential bipolar second stage output.

A third aspect of the invention is directed to a circuit to produce abipolar output signal from a circuit having at least two stages. Theapparatus includes a first stage of a circuit to produce a first stageoutput, when an input signal is coupled to the first stage output,wherein the first stage of the circuit includes one or more modulescoupled together to produce the first stage output; and a second stageof the circuit to produce a bipolar second stage output, wherein thesecond stage of the circuit includes one or more floating modulescoupled together to produce the bipolar second stage output.

A fourth aspect of the invention is directed to a circuit, having atleast two stages, to produce a differential bipolar output signal. Thecircuit includes a first stage of a circuit to produce a first stageoutput from an input signal, wherein the first stage of the circuitincludes one or more modules coupled together to produce the first stageoutput; and a second stage of the circuit to produce a differentialbipolar second stage output after applying the first stage output to asan input signal, wherein the second stage of the circuit includes one ormore floating modules coupled together to produce the differentialbipolar second stage output.

These and other objects and advantages of the invention will becomeapparent to those skilled in the art from the following detaileddescription of the invention and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the use of floating differential drive amplifiermodules coupled in series for a segmented high voltage amplifier, inaccordance with one embodiment of the invention.

FIG. 2 illustrates the use of a gain stage amplifier module and aunity-gain buffer stage module, in accordance with one embodiment of theinvention.

FIG. 3 illustrates the use of a gain stage amplifier including asymmetrical Class-A differential amplifier, in accordance with oneembodiment of the invention.

FIG. 4 illustrates a buffer amplifier for the output drive stage thatcontain a complimentary pair of complimentary emitter followers toprovide a high performance, no voltage gain, current amplifier, inaccordance with one embodiment of the invention.

FIG. 5 illustrates the use of a digital application input stage, inaccordance with one embodiment of the invention.

FIG. 6 illustrates an analog drive output module using isolationtransformers to communicate the analog audio signal in a transformerbased output module, in accordance with one embodiment of the invention.

FIG. 7 illustrates an output drive function split into two phases by atransformer based output module, in accordance with one embodiment ofthe invention.

FIG. 8 illustrates digital audio available as a source, where the inputvoltage isolation is provided with one or more digital opto-isolators inan opto-isolator based module, in accordance with one embodiment of theinvention.

FIG. 9 illustrates a digital correction circuit's topology, inaccordance with one embodiment of the invention.

FIG. 10 illustrates another embodiment with lookup table linearization,in accordance with one embodiment of the invention.

FIG. 11 illustrates a circuit to use inverse simulation linearization,in accordance with one embodiment of the invention.

FIG. 12 illustrates a Class AB amplifier, in accordance with oneembodiment of the invention.

FIG. 13 illustrates a flowchart of a method to produce a bipolar outputsignal from a circuit having at least two stages, according to oneembodiment of the invention.

FIG. 14 illustrates a flowchart of a method to produce anaudio-frequency bipolar output signal from a circuit having at least twostages, according to one embodiment of the invention.

FIG. 15 illustrates a flowchart of a method to produce a differentialbipolar output signal from a circuit having at least two stages,according to one embodiment of the invention.

FIG. 16 illustrates a flowchart of a method to produce anaudio-frequency differential bipolar output signal from a circuit havingat least two stages, according to one embodiment of the invention.

FIG. 17 illustrates a flowchart of a method to produce a substantiallylinear output signal from a circuit, according one embodiment of theinvention.

FIG. 18 illustrates a flowchart of a method to produce a substantiallylinear output signal from a circuit, according one embodiment of theinvention.

FIG. 19 illustrates a flowchart of a method to produce a substantiallylinear output signal from a circuit, according one embodiment of theinvention.

FIG. 20 illustrates a flowchart of a method to produce a substantiallylinear output signal from a circuit, according one embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention provides a method, an apparatus, and a system to obtainlow distortion amplification of signals (e.g., audio-frequency signals,higher frequencies, or equivalent signals requiring low distortionamplification. Various embodiments of the invention can be applied tobiological applications, medical applications, electronic applications,and any other applications where low distortion amplification can bebeneficially used and needed for driving capacitive loads. In thisspecification, drawings, and claims, any instance of the termaudio-frequency is defined as any signal frequency in the frequencyrange of 15 to 20,000 cycles per second (Hertz).

Other terms used below are defined as follows. Unipolar amplifier—Anamplifier that has a single power supply, where the amplifier has asingle output that is offset from the power supply rails. This outputtypically needs a blocking capacitor to provide an unbiased output withrespect to a power supply reference. Bipolar amplifier—An amplifier thathas the equivalent of two power supplies, one positive and one negativewith respect to a reference point. This layout allows for a singleoutput with no DC bias with respect to the power supply reference.Differential amplifier—An amplifier which has two outputs, one whichgoes positive with a positive output and one which goes the negativedirection with a positive output. When this amplifier is floating, thetwo output amplifiers can be either unipolar or bipolar, since they donot require a power supply reference to provide output power.

In one embodiment of the invention, a bipolar amplifier can stand-aloneand be combined with another inverting bipolar amplifier to produce adifferential output amplifier. Therefore, the modular approach startswith a single bipolar amplifier and can be expanded to paralleled and/orserialized segmented differential amplifiers. This allows buildingfloating bipolar amplifiers in various embodiments of the invention.

Solid-state amplifiers have not been used regularly for direct highvoltage drive of electrostatic loudspeakers. The drive voltagesnecessary for electrostatic speakers are generally around 2000 V_(RMS).Solid-state devices, such as transistors and MOSFETs, normally willwithstand a maximum of 1000 V_(DC). Various embodiments of the inventiondescribed here will economically produce almost arbitrarily large drivevoltages. It is also applicable for lower voltage high performanceamplifiers driving traditional loudspeakers.

FIG. 1 illustrates the use of floating differential drive amplifiermodules coupled in series for a segmented high voltage amplifier, inaccordance with one embodiment of the invention. Input signal 102 isinput to input stage 104, which in turn provides input signals to outputmodules 106, 108, 110, and 112, which in turn provide output positivepolarity output signal 114, negative polarity output signal 118, andground 118. With multiple isolated differential output modules wired inseries, an amplifier can provide a selected output voltage. With exactgain matching it is acceptable to wire multiple output modules inparallel to produce a selected output current. Since there is notraditional negative feedback, the output modules would be completelystable providing twice the output current. These groups of modules canbe wired in series, and parallel, to provide arbitrary voltage andcurrent performance to match any impedance and drive needs with lowvoltage and low current devices in alternative embodiments.

Traditional loudspeaker applications can benefit from this approach.Amplifiers can have arbitrary drive characteristics. In particular, astereo amplifier with two channels using output modules similar to thosein the above example, with correct switching can couple the variousoutput modules in series or parallel. As a result, the same amplifiercan be a stereo amplifier, a mono high current amplifier for lowerimpedances, or a mono high voltage amplifier for high impedance loads.This same function can be extended with several amplifier embodiments.This allows a truly universal amplifier module, which can be used inalmost any application, with a sufficient number of modules utilized.

These amplifier modules can be divided into two sections. The inputstage, which can be either analog or digital in design, provides inputinterfacing, processing, and possibly signal gain. The output stagecontains the output modules, which can also be driven by either analogor digital signals.

The analog input stage can be implemented using a very high performancegain-stage amplifier, a unity-gain buffer stage, a step-up transformer,or any combination of these. In any case it must properly drive theoutput modules of the design. A high performance input stage can beachieved using the following technology.

FIG. 2 illustrates the use of a gain stage amplifier module and aunity-gain buffer stage module, in accordance with one embodiment of theinvention. Input signal 102 and ground 116 are the inputs to input stageoperational amplifier 202, which in turn provides an input signal to aplurality of output modules 204, which in turn provides output positivepolarity output signal 114. In one embodiment of the invention, theinput gain stage amplifier is only used for voltage gain, and it isoptimized for high impedance resistive output loading. The unity-gainbuffer stage of the design is composed of a small to large number ofultra low distortion no-feedback buffer amplifiers wired in parallel.The input gain stage amplifier and the unity-gain buffer stage can becombined as shown in FIG. 2.

The voltage gain input amplifier can utilize many different traditionaltechnologies and topologies. Both solid state and tube circuits can beused for voltage amplification. Because this circuit drives the highimpedance inputs of the buffer amplifiers, the input voltage gain stageamplifiers can be very carefully tailored for linear voltage performanceinto a resistive load.

FIG. 3 illustrates the use of a gain stage amplifier including asymmetrical Class-A differential amplifier, in accordance with oneembodiment of the invention. Positive polarity input signal 102 andnegative polarity input signal 302 are inputs to JFETs 328 and 330,respectively. A positive voltage supply 304 and a negative polaritysupply 306 energize the amplifier circuit including resistors 320, 322,332, 334, 336, 340, and 342; JFETs 324, 326, 338, and current sources344 and 346, which in turn provide output signal 314. When matchedcomplimentary JFETs are used in the design, a very linear, highperformance voltage gain stage is achieved without negative feedback.

Any negative feedback used around this embodiment will only furtherlinearize the amplifier. Because this amplifier does not have slow highcurrent output devices in the feedback loop or high capacitive loading,there is very little propagation time through the amplifier, which canenlarge dither or overshoot effects in the signal.

FIG. 4 illustrates a buffer amplifier for the output drive stage thatcontain a complimentary pair of complimentary emitter followers toprovide a high performance, no voltage gain, current amplifier, inaccordance with one embodiment of the invention. Positive polarity inputsignal 102 is an input to the bases of bipolar transistors 406 and 420.A positive voltage supply 304 and a negative polarity supply 306energize the amplifier circuit including current sources 402 and 424;bipolar transistors 404, 406, 420; resistors 412 and 414; and diodegroups 408, 410, 416, and 418; which in turn provide output signal 114.Series resistors 412 and 414 are wired in the output of the outputtransistors 404 and 422 for linearization and output current limiting.These current limit resistors 412 and 414 protect the amplifiers, butprevent the high damping factors (low output impedance) needed for highfidelity operation. By utilizing a large number of these amplifiers inparallel the effective damping factor can be as high as that obtainedwith large amounts of negative feedback in traditional output circuits.Since each amplifier is relatively low power, the transistors in theamplifiers output stage have more ideal characteristics. In particular,the current in the collectors varies little with differences incollector voltages. As a result, power supply isolation and outputlinearity are much better than if a small number of large geometryoutput devices are used. The small geometry output devices also havemuch better high frequency performance than large geometry powerdevices. In one embodiment hundreds of these output buffers can be used.

A large number of buffer amps in parallel will also generate the veryhigh currents needed to drive large capacitive loads of electrostatictransducers and any step-up transformers used to generate the high drivevoltages required in this application. Because the buffer amplifiers canbe monolithic devices, the cost of the entire amplifier can becompetitive. Discrete device buffer amplifiers can also be utilized inalternative embodiments that are especially appropriate in high-voltagedirect drive applications.

In alternative embodiments, the output buffers can be implemented withconventional operational amplifiers wired in a unity-gain configuration.There are several monolithic amplifiers that have high output currentand acceptable performance, which can be suitable for this design. Thisconfiguration is also applicable in high voltage drive applicationswhere the dither of the unity-gain feedback can be proportionallysmaller because of the high drive voltages and the potentially smallamount of dither. These amplifiers might need a series resistor in theiroutput for stability when the amplifiers are coupled in parallel, but aswith the buffer amplifiers, when a large number of them are used theoutput impedance will drop to acceptable levels. The output impedancedetermines the damping factor of an amplifier, which is the ratio of theoutput impedance of the amplifier to the load impedance. A dampingfactor of 100 to 1 with an eight-ohm load means the effective outputimpedance of the amplifier is one-twelfth of an ohm. This is usuallyconsidered pretty good performance. The wire impedance connecting theload can be this high. High performance amplifiers can have dampingfactors as high as 200 or more.

If the input stage needs to provide higher output voltages, a low poweraudio step-up transformer can be used as a no-feedback voltage gainstage. The loading of the following output modules can be quite low(high impedance) so this transformer can be low power and highimpedance, which makes it easier to make, better performing, and lowercost. This transformer, with and without input and output bufferamplifiers, can be the only voltage gain of the amplifier. Since thistransformer can have multiple secondary winding outputs, this approachcan also provide the isolation needed for the output modules. The outputmodules could then only need to contain power supplies and bufferamplifiers.

An input stage can also consist of an ADC circuit which digitizes theanalog input and provides it through the digital input methods describedbelow to give digital processing advantages to a amplifier provided withanalog input voltages. The processed signal can be converted back toanalog to drive analog output modules or directly drive digital outputmodules mentioned below.

Digital Input Module

FIG. 5 illustrates the use of a digital application input stage, inaccordance with one embodiment of the invention. Network In 502 is aninput to Media Network Receiver 510; Digital Audio In 504 is an input toAES/EBU SPDIF Receiver 512; I2S In 506 is an input to I2S Interface 514;and Analog In 508 is an input to Analog-to-Digital Converter (ADC) 516.AES/EBU is an acronym for the Audio Engineering Society/EuropeanBroadcast Union, which established the basic professional stereo digitalaudio data distribution format. SPDIF is an acronym for the Sony PhilipsDigital Interface Format, which is the consumer version of the AES/EBUstandard. I2S is an acronym for IIS which is the nomenclature for aspecific internal digital audio data format. These modules in turnprovide inputs to Mux module 518, which provides an input to theEqualization module 520. Speaker Linearization module 522 receivesinputs from the Sensor In 528 and Equalization module 520, and providesan input to an Output Driver or a Digital-to-Analog Converter (DAC)w/Drive Amp module 524, which in turn provides an output 530. In digitalapplications, the input stage can provide:

AES/EBU, SPDIF and/or network data reception

Digital signal processing for equalization and crossover functions

Digital feedback and processing for speaker and amplifier correction

Drive circuitry for opto-isolator inputs of Digital Output Modules

DAC and drive electronics for Analog Output Modules

Several of these functions can also exist in the digital output modules.They are listed here for their global function.

The linearization processing techniques applicable here are mentionedbelow. They would be used here for linearizing loudspeakercharacteristics, for example. With a Digital Input Stage, the highvoltage DAC output module design described below in Digital OutputModules can be used as the output circuit for the Digital Input Stagewhen driving Analog Output Modules. A step-up transformer with multiplesecondary winding outputs connected to the analog output of this modulecan be used to help lower system costs, as mentioned above. Atraditional DAC design driving the Analog Input Stage mentioned abovewill also perform well as a Digital Input Stage driving Analog OutputModules.

In one embodiment, the output modules are electrically isolated from thechassis. The audio signal can be communicated to the module in eitheranalog or digital format. Either transformers or opto-isolators can actas isolation devices to provide the voltage isolation required foreither digital or analog amplifiers. In one embodiment, transformers arealso used to couple operating power into the modules and provide thenecessary voltage isolation.

Analog Output Module

FIG. 6 illustrates an analog drive output module using isolationtransformers to communicate the analog audio signal in a transformerbased output module, in accordance with one embodiment of the invention.Positive polarity input signal 102 and negative polarity input signal302 are inputs to transformer 608. Positive AC power source 602 andnegative AC power source 604 energize Power Supply +/−High Voltage 606,which provides positive and negative polarity voltages to amplifiers 610and 612, which provide positive and negative audio outputs 114 and 118,respectively.

This output module can have both voltage and current amplification, orjust current amplification. The most economical and highest performanceapproach is to have a common voltage gain amplifier for all the modulesand only perform current amplification in each output module. Voltageamplification can also be provided by a step-up input transformers whichwould provide both voltage gain and voltage isolation. This would allowthe common drive electronics in the input stage to run at lower voltagesand to relegate the potentially higher voltages to the output modules.There could be some performance degradation from this transformerstep-up function, but it would be much less than that of a standardelectrostatic step-up output transformer, because it does not have todrive high capacitive loads.

The current driving output section needs to be a very low distortion,very high speed, high voltage drive design that provides high outputcurrent. These four attributes can be very hard to provide at one time.Since the output amplifiers can be operational amplifiers wired asunity-gain devices, or can be single or paralleled high voltage versionsof the buffer amplifiers described above, this design can have lowartifacts normally not occurring in high voltage-gain circuits.

FIG. 7 illustrates an output drive function split into two phases by atransformer based output module, in accordance with one embodiment ofthe invention. Positive polarity input signal 102 and negative polarityinput signal 302 are inputs to transformer 608. Positive AC power source602 and negative AC power source 604 energize Power Supply +/−HighVoltage 606, which provides positive and negative polarity voltages toamplifiers 704 and 706. Power supply +/−15 V modules 702 and 708 providepositive and negative voltages to amplifiers 610 and 612, which providepositive and negative audio outputs 114 and 118, respectively.

The first stage is a very fast and high current amplifier. In effect, itprovides the voltage offset for the actual output drive circuit. Theoutput drive circuit consists of a second output circuit along with itspower supply. This second stage is a very fast and low distortion, lowvoltage buffer amplifier. This buffer amplifier is only used to providethe actual output signal over a very small voltage range. It can beidentical to the parallel buffer amp technology mentioned in the analoginput stage section above. Because this buffer is much faster than thevoltage drive amplifier that biases it, it is capable of correcting forany errors present in the high voltage drive section.

This buffer amp also has excellent common mode and power supplyrejection, which also helps isolate errors that the voltage drive ampmay have. One embodiment of the invention uses a buffer amp that doesnot utilize negative feedback, so there is no fidelity degradation fromfeedback hunting or overshoot even if the high voltage amplifier hasthese characteristics. A more traditional low voltage operationamplifier wired in a unity-gain configuration can also be used as thisbuffer amplifier. Multiples of them can be wired in parallel asdescribed previously.

Digital Output Module

FIG. 8 illustrates digital audio available as a source, where the inputvoltage isolation would be provided with one or more digitalopto-isolators in an opto-isolator based module, in accordance with oneembodiment of the invention. Digital Audio In signal 802 is an input toOpto-Isolator module 804, which provides an input to high voltage HVDACs module 806. Positive AC power source 602 and negative AC powersource 604 energize Power Supply +/−High Voltage 606, which providespositive and negative polarity voltages to HV DACs module 806 and toamplifiers 704 and 706. Power supply +/−15 V modules 702 and 708 providepositive and negative voltages to amplifiers 610 and 612, which providepositive and negative audio outputs 114 and 118, respectively.

With digital source material, the output module will be responsible forboth the conversion of the digital signals to analog and theamplification of this analog signal to the levels needed for the outputcurrent buffers. One embodiment would utilize a DAC and a traditionallow voltage power amplifier, like the one mentioned above for the analoginput stage. It would amplify the voltage and current of the DAC anddrive a step-up transformer for the bulk of the voltage gain required.As mentioned previously, this transformer would not have to drive heavyloads so it would not have to be as large and expensive as traditionaloutput transformers.

Digital Amplifier Linearization

A second approach would utilize the power of digital processing tocreate an amplifier design that is difficult to implement using othertechniques. One embodiment utilizes a digital lookup or error simulationcircuit to linearize a simple single active device voltage amplifierstage (this is only an example, because in other embodiments, a group ofmultiple amplifier devices can be linearized in a similar manner), whichhas very consistent and predictable distortion characteristics. Digitalpre-distortion of the input to the device is used to effectively correctthis device's inherent non-linearities. This approach does not usenegative feedback in the traditional sense, so it does not have thedetrimental effects on audio, mentioned above. In another embodiment itcan include a feedback mechanism, which continually monitors the outputof the amplifier to compensate for temperature, load, and aging drift ofthe device. The voltage amplifier device in various embodiments is abipolar transistor, MOSFET device, or a vacuum tube. The correctioncircuit can correct for linearity and time-based devices likecapacitance and non-linear capacitance. In one embodiment, thecorrection circuit can correct multiple grouped devices.

FIG. 9 illustrates a digital correction circuit's topology, inaccordance with one embodiment of the invention. Digital Audio inputsignal 504 is an input to the Linearization Logic module 906 and theInversion and Linearization Logic module 924. Linearization Logic module906 is coupled to DAC 908, ADC 918 and ADC 919. Inversion andLinearization Logic module 924 is coupled to DAC 926, ADC 934 and ADC935. Output buffers 910 and 928 are coupled to DAC 908 and DAC 926,respectively. Input buffers 920, 921, 936 and 937 are coupled to ADC918, ADC 919, ADC 934 and ADC 935, respectively. A positive voltagesupply 902 and a negative polarity voltage supply 916 energize theamplifier circuit including transistors 912 and 930; and resistors 904,905, 914, 915, 922, 923, 932, and 933; which in turn provide positiveand negative polarity output signals 114 and 118. The resistors 904,905, 914, 915, 922, 923, 932, and 933 provide linearization and outputcurrent limiting.

In another embodiment, voltage divider resistors 905, 915, 923, and 933can be deleted, and the input to buffer 921 can be directly connected tothe positive polarity output signal 114, and the input to buffer 937 canbe directly connected to the negative polarity output signal 118. Thefeedback circuit consists of ADCs 918, 919, 934, and 935 having aresolution equal to or higher than the resolution needed in theapplication. This embodiment provides an output monitor signal to acomparison circuit that compares it with the properly delayed DAC inputsignal and generates a difference value. In one embodiment, there wouldbe monitoring ADC converters for both output voltage and current.

Lookup Table Linearization

FIG. 10 illustrates another embodiment with lookup table linearization,in accordance with one embodiment of the invention. Control Processormodule 1004 receives input signal 1002 and digital audio input signal504; and provides inputs to the Dual Port RAM Lookup Table modules 1006and 1014. Amp Scale modules 1008 and 1016 receive inputs from PanControl module 1010, and produce an output signal from the Summingmodule 1012, which produces the audio output 114. Here the differencevalue discussed above, or a fraction of it, can be added into a RAMlookup correction table residing between the source data and the DAC.

If the values are simply inserted in the active lookup table, there willbe glitches in the output signal. If two lookup tables are used with asimple gradual “pan” between the two outputs of the two tables, thenthere will be no discontinuities on the output when the switch occurs. Apan is achieved by the gradual shifting of complimentary amplitudescaling of the two outputs and final summing. This pan has to occurslowly enough so that it does not introduce an audible thump. In oneembodiment of the invention, a pan time of a major part of a second isusually appropriate and sufficient.

When a lookup table is updated it will be for a small number of valuesof the table. If only a single value is changed, then a glitch in theoutput will occur when the signal references this point. With the use ofa smoothing operation like a quadratic or spline function, the table andall its neighboring values can be updated while maintaining a continuousfunction in the table. The nature of this smoothing can be varied fordifferent parts of the table. In some embodiments, higher performance isachieved by shifting the whole table by a constant to offset it, ratherthan by updating small parts of the table. For example, this isappropriate when compensating for temperature shifts of deviceparameters. The net effect of implementing all of these techniques is tocreate a new table using a few points, such that the table develops amore accurate overall performance.

Inverse Simulation Linearization

An alternative embodiment would include an inverse simulation model ofthe individual elements of the device in the circuit to be corrected. Bypassing the input signal through this model before passing it on to theDAC and the device, the negative effects of the device can effectivelybe removed. The full inverse simulation would usually occur for eachsample of the audio.

FIG. 11 illustrates a circuit to use inverse simulation linearization,in accordance with one embodiment of the invention. The inversesimulation can be performed using a control processor (e.g., a DSP, ageneral-purpose processor, or an equivalent processor). ControlProcessor module 1004 receives sensor input signal 1100 and digitalaudio input signal 504; and provides inputs to the Device ThermalSimulation module 1102, Heatsink Thermal Simulation module 1104, AmpScale module 1008, Offset module 1106, Dual Port RAM Lookup Table module1014, Amp Scale modules 1016, Offset module 1108, DeviceOutputCapacitance Compensation Equalization module 1110, and Device InputCapacitance Compensation Equalization module 1112. Modules 1108, 1110,and 1112 provide inputs to the Summing module 1012, which produces theaudio output 114.

In one embodiment, an inverse simulation model consists of an exactcompliment of each of the simulation devices used to normally simulatethat device in an engineering simulation program (e.g., such as SPICE,or other equivalent programs). Many correction elements can also beperformed by signal processing techniques. As a result, any distortioneffects, capacitances, inductances, or resistances intrinsic in the partwould be compensated for by the inverse functions in the inverse model.By slowly adjusting each of the parameters of the inverse model andmonitoring the actual device output as described above, it is possibleto minimize the amplifier distortion.

In one embodiment, an annealing effect can be applied by scaling thiscontinual adjustment. The annealing process is achieved by initiallyproviding large changes (“high heat”) to the correction. As theperformance improves, the circuit reduces the size of the corrections(“lower heat”). In this embodiment, when the amplifier starts up, theseparameters could be adjusted over a relatively wide range. As theamplifier error becomes less, then the size of the variations isdecreased until the parameter variations are adequate to maintain a verylow distortion. Initial values can be remembered from previouscorrections and preset into the circuit on startup so that thecorrection process is always adjusting for smaller variations in themodel. Initial start up parameters can also be generalized or rememberedfrom actual measurements of the amplifying device during unitsmanufacturing and calibration.

The simulation illustrated in the figure above shows many of theelements needed for simulating devices such as bipolar transistors orMOSFETS. The input scaling and offset units are set by the ControlProcessor to modify the input signal and match it to the log or squareroot like function loaded into the lookup table. The function in thetable can also be updated, but many applications only need to modify thetable access and global parameters of the data coming from the table.The scaling and offset factors on the data read from the table helpmatch the table's contents to the particular device in the circuit. Thetwo equalized and delayed signals that are summed with the table outputcompensate for the effects of the internal device capacitances. Thesecapacitances will remove high frequency energy. The two band-passfilters add that energy into the input of the device to compensate forand remove the effect. The two thermal simulation modules compute thepower dissipation of the device and the temperature influences theheating of the chip and package will have on the operation of thedevice. The Control Processor can modify the inverse simulationparameters to compensate for the device parameter changes as it is beingheated by its operation.

Composite Amplifier Topologies

The linearized amplifier with isolation disclosed above can be used asan active device in various amplifier topologies. The amplifierdiscussed above is a simple Class A design. In one embodiment, two ofthese circuits wired in series between two power supply sources, drivenwith a coordinating digital processing section, can be used as apush-pull Class B or Class AB design.

FIG. 12 illustrates a Class AB amplifier, in accordance with oneembodiment of the invention. Digital Audio input signal 504 and outputvoltage and current are inputs to the Output Control module 1202, whichprovides inputs to the Linearization Logic module 906 and theLinearization Logic module 1204. The output voltage and currents aremeasured by ADC converters integrated with the Output Control module1202 and the Linearization Logic module 906. Linearization Logic module906 is coupled to DAC 908 and ADC 918. Linearization Logic module 1204is coupled to DAC 926 and ADC 934. Output buffers 910 and 928 arecoupled to DAC 908 and DAC 926, respectively. Input buffers 920 and 936are coupled to ADC 918 and ADC 934, respectively. A positive voltagesupply 902 and a negative polarity voltage supply 916 energize theamplifier circuit including transistors 912 and 930; resistors 904, 914,922 and 932; which in turn provide positive and negative polarity outputsignals 114 and 118.

The monitoring of the performance of the linearized device gives thecorrection circuit the actual output current of the circuit element. Thecoordinating Output Control circuitry would only have to specify theinstantaneous current of the linearized device at a regular interval tocoordinate its contribution to the circuit. In various embodiments,these linearized devices can be wired in series to provide highervoltage operation, and/or they can be wired in parallel to deliverhigher drive current. In one embodiment, the contributions of variousdevices can be switched on and off, as the need exists to reduce powerdissipation or dynamically improve speed.

Loudspeaker Linearization

In one embodiment of the invention, the linearization techniquesdisclosed above can also be applied to a loudspeaker. A sensor, ormicrophone built into the loudspeaker can give feedback to thelinearization circuits similar to those discussed above. Measurements ofspecific parameters of the speaker, like diaphragm displacement, mightbe used as a stimulus for a global distortion correction circuit.

One issue that occurs when measuring speaker position for correctionsystems is the potential interference from ambient room noise and otherspeakers. In one embodiment, one solution to this problem is to use adigital convolver to extract the source audio from the returned signalfrom the sensor. Accumulating the products of the two signals over asequence of samples performs the convolution. When the input signal iscorrectly synchronized with the return signal, the sum of the productswill increase dramatically. The height of this sum gives the correlationof the input and output signals.

In general, this sum of products does not give much more informationthan the amplitude of the detected signal. If the source signal isband-pass filtered before the convolution, then the height of thecorrelation will give the amplitude of that band of the signal. By usingvarious band-pass center frequencies and bandwidths, the overallfrequency response of the speaker can be determined. Since the responseof the speaker is in many cases related to the room response, thiscorrelated response detection can also help correct for room responses.This is easier to do if the actual response of the speaker is measuredduring manufacturing in a neutral room and this response is comparedwith the sensed return.

In many situations, the distortion of the speaker increases as theamplitude increases. This normally occurs only at lower frequencies. Inone embodiment, by band-pass filtering the input signal and thenfrequency multiplying the resultant signal by two and three times, theconvolution will allow the harmonic distortion to be measuredselectively at second and third harmonics. By comparing thesecorrelations at different levels, the distortion in regard to amplitudecan be accurately measured. In one embodiment, this distortionmeasurement can be used to modify an inverse model simulation of theloudspeaker to pre-distort the signal to eliminate the effect of thetransducers nonlinearities. By using the annealing technology disclosedabove, this correction can be done continually while the speaker isplaying.

In one embodiment of the invention, this correction can be utilized forcorrecting distortions created by manufacturing errors and for basictransducer nonlinearity characteristics. In order to correct for theseerrors, substantially exact speaker diaphragm displacement mustsometimes be determined. In one embodiment, this is performed by lookingat the second and third harmonic distortion of the speaker as theamplitude changes. Second harmonic distortion sometimes comes fromasymmetric drive. Third harmonic distortion can come from clipping orsymmetric distortion. Since the actual displacement is the result of thedrive voltages, cabinet, and ambient influences, it cannot be determinedjust by dynamic modeling. In one embodiment, the accurate measurement ofthe distortion using the convolution techniques disclosed above can beused to verify and correct the simulation of the diaphragm.

Digital Clocking

For the best fidelity, the clock used by the output DAC must beextremely stable. There are several approaches to stabilize this clock.One embodiment would use a dedicated reference oscillator on each outputmodule along with a sample rate converter. The sample rate converterswould need to be synchronized at startup to lock the audio signalsynchronization between all of the modules. Another embodiment would usea clock transmitted from a stable input clock to each module by ahigh-speed opto-isolator designed to introduce a minimum amount ofjitter.

Isolation Capacitance

Each of the output modules needs their own isolating power sources. Thiscan be achieved with 60/50 Hz line frequency power transformers,standard bridge rectifiers, capacitors, and analog voltage regulation.Another embodiment would use a switching power supply with a highfrequency isolation transformer. The output voltage can be regulatedwith the switching circuitry. Switching power supplies would be smaller,lighter, and could have better low frequency performance. The smallertransformer used in switching power supplies would have lowercapacitance between windings, and it might be easier to make thiscapacitance highly linear.

With all of the voltage isolation techniques used in these amplifierapproaches, it is important to design them to have extremely low andstable coupling capacitance. All of this capacitance will appear to bein parallel with the electrostatic transducer capacitance. Most speakersat the present time have a capacitance of around 1000 pf. All of thecapacitance from the isolation devices should add up to less than thisamount. Any non-linearities in this capacitance, which is a function ofchanges in capacitance as the applied voltage changes, will causenonlinear loading on the output of the amplifier. This loading can beovercome by higher damping factors and higher current drive of theoutput modules, and by careful design of the isolation circuitry toeliminate these variations.

FIG. 13 illustrates a flowchart of a method to produce a bipolar outputsignal from a circuit having at least two stages, according to oneembodiment of the invention. The sequence starts in operation 1302.Operation 1304 includes coupling an input signal to a first stage of acircuit to produce a first stage output, wherein the first stage of thecircuit includes a first plurality of modules coupled together toproduce the first stage output. Operation 1306 includes applying thefirst stage output as an input signal to a second stage of the circuitto produce a bipolar second stage output, wherein the second stage ofthe circuit includes a second plurality of floating modules coupledtogether to produce the bipolar second stage output. The method ends inoperation 1308.

FIG. 14 illustrates a flowchart of a method to produce anaudio-frequency bipolar output signal from a circuit having at least twostages, according to one embodiment of the invention. The sequencestarts in operation 1402. Operation 1404 includes coupling an inputsignal to a first stage of a circuit to produce a first stage output,wherein the first stage of the circuit includes a first plurality ofmodules coupled together to produce the first stage output. Operation1406 includes applying the first stage output as an input signal to asecond stage of the circuit to produce an audio-frequency bipolar secondstage output, wherein the second stage of the circuit includes a secondplurality of floating modules coupled together to produce theaudio-frequency bipolar second stage output. The method ends inoperation 1408.

FIG. 15 illustrates a flowchart of a method to produce a differentialbipolar output signal from a circuit having at least two stages,according to one embodiment of the invention. The sequence starts inoperation 1502. Operation 1504 includes coupling an input signal to afirst stage of a circuit to produce a first stage output, wherein thefirst stage of the circuit includes a first plurality of modules coupledtogether to produce the first stage output. Operation 1506 includesapplying the first stage output as an input signal to a second stage ofthe circuit to produce a differential bipolar second stage output,wherein the second stage of the circuit includes a second plurality offloating modules coupled together to produce the differential bipolarsecond stage output. The method ends in operation 1508.

FIG. 16 illustrates a flowchart of a method to produce anaudio-frequency differential bipolar output signal from a circuit havingat least two stages, according one embodiment of the invention. Thesequence starts in operation 1602. Operation 1604 includes coupling aninput signal to a first stage of a circuit to produce a first stageoutput, wherein the first stage of the circuit includes a firstplurality of modules coupled together to produce the first stage output.Operation 1606 includes applying the first stage output as an inputsignal to a second stage of the circuit to produce an audio-frequencydifferential bipolar second stage output, wherein the second stage ofthe circuit includes a second plurality of floating modules coupledtogether to produce the audio-frequency differential bipolar secondstage output. The method ends in operation 1608.

FIG. 17 illustrates a flowchart of a method to produce a substantiallylinear output signal from a circuit, according one embodiment of theinvention. The sequence starts in operation 1702. Operation 1704includes utilizing a linearization module (e.g., including one or moredigital lookup tables, an error simulation circuit, or an equivalent) tolinearize at least one device with at least one substantiallypredictable distortion characteristic. Operation 1706 includespre-distorting at least one input to the at least one device tosubstantially correct at least one parameter of the at least one deviceto produce a substantially corrected output. The method ends inoperation 1708.

If the values are simply inserted in the active lookup table, there willbe glitches in the output signal. If two lookup tables are used with asimple gradual “pan” between the two outputs of the two tables, thenthere will be no discontinuities on the output when the switch occurs.Panning is achieved by the gradual shifting of complimentary amplitudescaling of the two outputs and final summing. This panning has to occurslowly enough so that it does not introduce an audible thump. When alookup table is updated it will be for a small number of values of thetable. If only a single value is changed, then a glitch in the outputwill occur when the signal references this point. By smoothing (e.g,using a quadratic or spline function), the table and all its neighboringvalues can be updated while maintaining a continuous function in thetable. The nature of this smoothing can be varied for different parts ofthe table.

FIG. 18 illustrates a flowchart of a method to produce a substantiallylinear output signal from a circuit, according one embodiment of theinvention. The sequence starts in operation 1802. Operation 1804includes utilizing a linearization module (e.g., including a digitallookup table, an error simulation circuit, or an equivalent) tolinearize at least one parameter of at least one device in said circuit,wherein said at least one device has at least one substantiallypredictable parameter distortion characteristic. Operation 1806 includespre-distorting at least one input signal to said at least one device toeffectively correct said at least one substantially predictableparameter of said at least one device to produce a substantiallycorrected output. Operation 1808 includes utilizing a feedbackmechanism, which monitors said substantially corrected output of said atleast one device to compensate for changes in said at least onesubstantially predictable parameter of said at least one device (e.g.,due to temperature, load, aging drift, or another equivalent factor).The method ends in operation 1810.

FIG. 19 illustrates a flowchart of a method to produce a substantiallylinear output signal from a circuit, according one embodiment of theinvention. The sequence starts in operation 1902. Operation 1904includes utilizing an inverse simulation model of a device in a circuitto be corrected, wherein by passing an input signal through the inversesimulation model before passing it on to a DAC and the device, one ormore distortion effects of the device can substantially be removed.Operation 1906 includes utilizing a control processor module to receiveat least one sensor input signal and a digital audio input signal.Operation 1908 includes providing a plurality of signals from thecontrol processor module to at least one module (e.g., such as asimulation module, an amp scale module, an offset module, a dual portlookup table module, and/or a device compensation equalization module)to produce a substantially corrected output. The method ends inoperation 1910.

FIG. 20 illustrates a flowchart of a method to produce a substantiallylinear output signal from a circuit, according one embodiment of theinvention. The sequence starts in operation 2002. Operation 2004includes utilizing an inverse simulation model of a device in a circuitto be corrected, wherein by passing the input signal through the inversesimulation model before passing it on to a DAC and the device, one ormore distortion effects of the device can substantially be corrected.Operation 2006 includes performing one or more signal processingtechniques to compensate for one or more distorting parameters (e.g.,distortion effects, capacitances, inductances, resistances, or anotherequivalent parameter) in the device by the inverse functions in theinverse simulation model, and adjusting one or more of the parameters ofthe inverse model and monitoring the actual device output to reducedistortion in the circuit by making a correction. Operation 2008includes applying an annealing effect by scaling the adjustment (whichcould be substantially continuous in one embodiment), wherein theannealing process initially provides large changes (“high heat”) to thecorrection, and as the performance improves, the circuit reduces thesize of the correction (“lower heat”). The method ends in operation2010.

In summary, one embodiment of the invention uses differentialamplifiers. And the amplifier outputs can go both positive and negativewith respect to each other. With conventional designs using transistorsor FETs, the collector/anode has to stay more positive than theemitter/drain for the amplifier to function. In one embodiment of theinvention, since each output can go positive and negative, it ispossible to provide a real differential bipolar output, rather than asingle unipolar one. An alternative embodiment uses a unipolar output,but it would require a series capacitor that would limit the outputsignal fidelity. In another embodiment of the invention, one or morecircuit stages are not amplifiers and are instead unity-gain buffers oran equivalent. The use of unity-gain buffers improves the performance ofthe circuit, since the linearity is very high. Another embodiment ofinvention uses one or more stages with amplifiers having a gain greaterthan unity, either in voltage or in current.

Several embodiments of the invention are possible. The phrase “in oneembodiment” used in the specification can refer to a new embodiment, adifferent embodiment disclosed elsewhere in the application, or the sameembodiment disclosed earlier in the application. The exemplaryembodiments described herein are for purposes of illustration and arenot intended to be limiting. Therefore, those skilled in the art willrecognize that other embodiments could be practiced without departingfrom the scope and spirit of the claims set forth below.

1. A method to produce a substantially linear output signal from acircuit, said method comprising: utilizing a linearization module tolinearize at least one parameter of at least one device in said circuit,wherein said at least one device has at least one substantiallypredictable parameter distortion characteristic; pre-distorting an inputsignal to said at least one device to substantially correct said atleast one substantially predictable parameter of said at least onedevice to produce a substantially corrected output; and utilizing afeedback mechanism, which monitors said substantially corrected outputof said at least one device to compensate for changes in said at leastone substantially predictable parameter of said at least one device toproduce a substantially linear output signal.
 2. The method of claim 1,wherein said at least one substantially predictable parameter includes aparameter selected from the group of parameters consisting ofcapacitances, inductances, resistances, parameters varying by atemperature of said at least one device, parameters varying by a currentsource or sink on said at least one device, parameters varying by avoltage differential on said at least one device, and parametersdrifting by an aging of said at least one device.
 3. The method of claim1, further comprising: using a plurality of lookup table entries forsaid pre-distorting at least one input; and panning of said plurality oflookup table entries to substantially eliminate a second plurality ofsingle value correction adjustment glitches to produce a substantiallylinear output signal.
 4. The method of claim 1, wherein saidsubstantially linear output signal includes at least one audio-frequencysignal.
 5. The method of claim 1, wherein said substantially linearoutput signal is provided by at least one differential amplifier.
 6. Themethod of claim 1, wherein said substantially linear output signal isprovided by at least one bipolar amplifier.
 7. A method to produce asubstantially linear output signal from a circuit, said methodcomprising: utilizing an inverse simulation model of at least one devicein a circuit to be corrected, wherein by passing an input signal throughsaid inverse simulation model before providing said input signal as aninput to a DAC and said at least one device, one or more distortioneffects of said at least one device can substantially be corrected;utilizing a control processor module to receive at least one sensorinput signal and a digital audio input signal; and providing a pluralityof signals from said control processor module to at least one outputmodule to produce a substantially corrected output signal.
 8. The methodof claim 7, wherein said one or more distortion effects are caused by aparameter selected from the group of parameters consisting ofcapacitances, inductances, resistances, parameters varying by atemperature of said at least one device, parameters varying by a currentsource or sink on said at least one device, parameters varying by avoltage differential on said at least one device, and parametersdrifting by an aging of said at least one device.
 9. The method of claim7, further comprising: using a plurality of lookup table entries forsaid inverse simulation model for pre-distorting at least one input; andpanning of said plurality of lookup table entries to substantiallyeliminate a second plurality of single value correction adjustmentglitches to produce a substantially linear output signal.
 10. The methodof claim 9, wherein said substantially linear output signal is providedby at least one differential amplifier.
 11. The method of claim 9,wherein said substantially linear output signal is provided by at leastone bipolar amplifier.
 12. The method of claim 7, further comprisingoutputting a substantially linear output signal that includes at leastone audio-frequency signal.
 13. The method of claim 7, wherein said atleast one device having one or more distortion effects includes at leastone transducer for a loudspeaker.
 14. A method to produce asubstantially linear output signal from a circuit, said methodcomprising: utilizing an inverse simulation model of at least one devicein a circuit to be corrected, wherein by passing said input signalthrough said inverse simulation model before providing said input signalas an input to a DAC and said at least one device, one or moredistortion effects of said at least one device can substantially becorrected; performing one or more signal processing techniques tocompensate for one or more distorting parameters in said at least onedevice by one or more inverse functions in said inverse simulationmodel, and adjusting one or more parameters of said inverse model andmonitoring said at least one device output to reduce distortion in saidcircuit by making a correction; and applying an annealing process byscaling said adjustment, wherein said annealing process initiallyprovides larger correction, and as performance of said circuit improves,said circuit reduces said correction to produce a substantially linearoutput signal.
 15. The method of claim 14, further comprising: using aplurality of lookup table entries for said inverse simulation model tocompensate for one or more distorting parameters by making saidcorrection; and panning of said plurality of lookup table entries tosubstantially eliminate a second plurality of single value correctionadjustment glitches to produce a substantially linear output signal. 16.The method of claim 15, wherein said substantially linear output signalis provided by at least one differential amplifier.
 17. The method ofclaim 15, wherein said substantially linear output signal is provided byat least one bipolar amplifier.
 18. The method of claim 15, wherein saidat least one device having one or more distortion effects includes atleast one transducer for a loudspeaker.
 19. The method of claim 14,wherein said one or more distortion effects are caused by a parameterselected from the group of parameters consisting of capacitances,inductances, resistances, parameters varying by a temperature of said atleast one device, parameters varying by a current source or sink on saidat least one device, parameters varying by a voltage differential onsaid at least one device, and parameters drifting by an aging of said atleast one device.
 20. The method of claim 14, further comprisingoutputting a substantially linear output signal that includes at leastone audio-frequency signal.